Conventionally, an automatic transmission for a vehicle has ranges P (parking), R (reverse), N (neutral), D (drive), S (second) and L (low) so that the gear ratio can be automatically changed when a driver selects any of the above-described ranges.
For example, in an automatic transmission of the type provided with five forward speed ranges and one reverse speed range, when a driver selects the D-range, in accordance with the vehicle speed and the degree of opening of the throttle, the gear ratio is automatically changed within a range between the first speed and the fifth speed. Furthermore, when the driver selects the S-range, the gear ratio is automatically changed in a range between the first speed and the third speed. Similarly, when the driver selects the L-range, the gear ratio is automatically changed in a range between the first speed and the second speed.
The range is selected in a conventional automatic transmission by an operator by moving the shift lever to one of a number of linearly disposed shift positions. That is, any of the shift positions arranged in the form of an I-shape can be selected.
However, the above-described conventional automatic transmission allows the gear ratio to be automatically switched in accordance with the operating conditions of the vehicle after a driver has once selected the range by moving the shift lever. Therefore, the desired gear ratio cannot be selected. Furthermore, a variety of gear ratios may be desired by drivers even under the same set of operating conditions. Therefore, the gear ratio which has been automatically selected by the automatic transmission is not always the gear ratio desired by the driver.
Another problem arises in that the conventional transmission cannot be operated with the feel of shifting feeling since the shift lever is moved linearly. Furthermore, since it has been difficult to recognize the selected shift position during the operation, a need exists for means capable of preventing an erroneous operation.
Therefore, there has been disclosed an automatic transmission for a vehicle in Japanese Patent Laid-Open No. 60-8912 which is arranged in such a manner that the gear ratio of the transmission can be fixed to that selected by a driver.
The above-described automatic transmission for a vehicle has been arranged in such a manner that the shift lever can be moved longitudinally and laterally so that shift positions arranged in the form of an H-shape can be selected in a manner similar to a manual transmission. Furthermore, there is provided a switch which is switched on when the shift lever is positioned at any of the shift positions so that a desired range and gear ratio can be selected in accordance with the position of the switch.
Another automatic transmission for a vehicle has been disclosed in Japanese Patent Laid-Open No. 61-157855 which has shift positions arranged in the form of an H-shape that can be selected in the manner of a manual shift in addition to the shift positions arranged in the form of the I-shape so used in the conventional automatic transmission.
In this case, when a shift position is selected from those arranged in the form of the H-shape, the switch disposed at the corresponding shift position is switched on so that a desired gear ratio is selected. On the contrary, when a shift position is selected from those arranged in the form of the I-shape, a desired range is selected.
In the thus structured automatic transmission for a vehicle, since a driver is able to select a desired shift position from those arranged in the form of the H-shape, the driver can optionally change the range and the gear ratio. Therefore, the feeling of manual shifting is obtained and erroneous operation can be prevented. In addition, direct shifting from a certain range or a gear ratio to another range or gear ratio is enabled, thus enabling the vehicle operation states to be varied.
In the above-described technology, the driver is able to select a shift position. However, in a case where jump shifting is performed in an automatic transmission provided with the low to the fifth (top) speeds, for example, when the vehicle speed is shifted from a first speed to the third speed or from the top speed to the third speed, the rotational speed input to the automatic transmission is considerably changed. As a result, an undesirable shock occurs due to the transmission operation, causing the life of each of the frictional engagement elements to be shortened.
As described above, since jump shifting has a multiplicity of problems, it must be prevented by some measures. It might therefore be considered feasible to employ a structure arranged in such a manner that the gear ratio is changed to the desired gear ratio via an intermediate gear ratio if the gear ratio next in succession is not selected, whereby the jump shifting can be prevented. However, it creates an excessively long time between the time the gear ratio is selected and the time the selected gear ratio is established.
In none of the above-described automatic transmissions, is any of the intermediate positions between the shift positions arranged in the form of the H-shape arranged to be the position at which a desired range or a gear ratio is selected. Usually, the gear ratio prior to the change of the shift position is maintained at the above-described intermediate position in order to stabilize operation.
Therefore, when a driver wants, during driving, to switch from the automatic transmission mode to the manual transmission mode and to drive the vehicle at a certain gear ratio, the shift lever must be moved through the intermediate position before switching to the desired gear ratio will occur.
In addition, in a case where the driver wants to cancel the manual transmission mode during driving, it is necessary for the driver to move the shift lever to a predetermined shift position which corresponds to an automatic transmission range in the form of the H-shape pattern.
As described above, in either of the case in which the manual transmission mode is changed to the automatic transmission mode or the case in which the automatic transmission mode is changed to the manual transmission mode, it is necessary for the driver to temporarily place the shift lever to the intermediate position, complicating the operation to be performed by the driver.
An object of the present invention is to provide an automatic transmission for a vehicle arranged in such a manner that the range and the gear ratio can be freely changed by the driver, the feel of shifting can be obtained, a desired range or a gear ratio can be directly selected from a certain range or a gear ratio so that the driving state can be varied. Another object is that, even if a discontinuous gear ratio is selected, generation of an excessively large shock within the transmission operation can be prevented, and the life of each of the frictional engagement elements is not shortened. A further object is to assure that shifting to the selected gear ratio is quickly established while providing a feeling of shifting as in the case of a manual transmission.
Another object of the present invention is to provide an automatic transmission for a vehicle capable of immediately changing a certain gear ratio during the vehicle driving in the manual transmission mode to the automatic transmission mode or changing the automatic transmission mode to the manual transmission mode.